U.S. Pat. No. 5,296,702 discloses techniques whereby an apparatus transmits radiation toward an object, receives the reflected radiation, and thereby obtains a spectral reflectance characteristic of the object. Because the spectral reflectance characteristic of a living plant differs from that of soil, weeds growing in a field can be differentiated from soil in the field. When the apparatus detects a spectral reflectance characteristic of a living plant, a high speed solenoid valve is opened allowing herbicide to flow to a spray nozzle and to spray the weed. When the apparatus no longer detects a spectral characteristic of a living plant, the solenoid valve is closed and herbicide is not sprayed onto the bare soil. Use of such an apparatus in spraying weeds in a field results in considerable herbicide savings because herbicide is not wasted on the bare soil.
FIG. 1 (Prior Art) is a simplified block diagram of a sprayer apparatus 1 which puts into practice the above described technique. Herbicide 2 is pumped by a pump 3 from a reservoir 4, through tubing 5, to a plurality of electronically controlled solenoid valves 6 and spray nozzles 7. A system filter 8 (for example, a 50 to 100 mesh filter) in a filter housing 9 is disposed between the reservoir and the pump to prevent particles from passing into the pump and from clogging the solenoid valves and spray nozzles.
When farmers used the sprayer apparatus, however, problems were encountered. Water (for example, water obtained from a ditch which may have appeared clear to the farmer) was put into the reservoir and was mixed with herbicide concentrate to make the needed liquid herbicide. When the sprayer apparatus stopped operating properly due to inadequate flow of herbicide to the spray nozzles, it was determined, perhaps due to the water having appeared clear when it was put in the reservoir, that the filter 8 could be removed and the sprayer would continue to operate. The system filter 8 was removed from housing 9 and spraying was resumed without system filter 8. Unfortunately, small particles present in the water then clogged the solenoid valves. Considerable effort was then required to clean the numerous solenoid valves. In some cases, catastrophic failures of solenoid valves occurred and the solenoid valves had to be replaced.